1. Field of the Invention
The present invention relates to a semiconductor element and its manufacturing method, particularly to a functional semiconductor element such as a photovoltaic element and a thin film transistor and its manufacturing method.
2. Related Background Art
Microcrystalline silicon semiconductors have been presented since 1979. See, e.g., S. USUI and M. KIKUCHI, "PROPERTIES OF HEAVILY DOPED GD-Si WITH LOW RESISTIVITY", Journal of Non-Crystalline Solids, 34 (1979), pp. 1 to 11. This article described that a low-resistivity microcrystalline silicon semiconductor doped with phosphorous was able to be deposited by a glow discharge method.
The same fact is described in A. MATSUDA, S. YAMASAKI et al., "Electrical and Structural Properties of Phosphorous-Doped Glow-Discharge Si:F:H and Si:H Films", Japanese Journal of Applied Physics, Vol. 19, No. 6, JUNE, 1980, pp. L305 to L308.
Further, A. Matsuda, M. Matsumura et al., "Boron Doping of Hydrogenated Silicon Thin Films", Japanese Journal of Applied Physics, Vol. 20, No. 3, MARCH, 1981, pp. L183 to L186 discusses the characteristics of a mixed-phase of boron-doped amorphous and microcrystalline silicon.
A. MATSUDA, T. YOSHIDA et al., "Structural Study on Amorphous-Microcrystalline Mixed-Phase Si:H Films", Japanese Journal of Applied Physics, Vol. 20, No. 6, JUNE, 1981, pp. L439 to L442 discusses the structure of an amorphous and microcrystalline mixed-phase.
However, the possibility that such mixed layers of amorphous and microcrystalline silicon could be applied to semiconductor elements such as solar cells has been suggested, but there has been no actual application.
Solar cells using microcrystalline silicon semiconductors have been described in U.S. Pat. No. 4,600,801 "FLUORINATED P-DOPED MICROCRYSTALLINE SILICON SEMICONDUCTOR ALLOY MATERIAL", U.S. Pat. No. 4,609,771 "TANDEM JUNCTION SOLAR CELL DEVICES INCORPORATING IMPROVED MICROCRYSTALLINE P-DOPED SEMICONDUCTOR ALLOY MATERIAL", and U.S. Pat. No. 4,775,425 "P- AND N-TYPE MICROCRYSTALLINE SEMICONDUCTOR ALLOY MATERIAL INCLUDING BAND GAP WIDENING ELEMENTS, DEVICES UTILIZING SAME". However, the microcrystalline silicon semiconductors described in these patents have been used in p-type or n-type semiconductor layers in solar cells of a pin structure using an amorphous i-type semiconductor layer.
Recently, articles on solar cells using microcrystalline silicon in an i-type semiconductor layer have been published. For example, there is "ON THE WAY TOWARDS HIGH EFFICIENCY THIN FILM SILICON SOLAR CELLS BY THE MICROMORPH CONCEPT", J. Meier, P. Torres et al., Mat. Res. Soc. Symp. Proc., Vol. 420, (1996) p. 3. However, as acknowledged by the authors of the article, the initial photoelectric conversion efficiency in a single structure solar cell using microcrystalline silicon in an i-type semiconductor layer is 7.7%, which is lower than that for solar cells with the same structure using amorphous silicon.
The present inventors have diligently inspected the reason why the conversion efficiency of solar cells using microcrystalline silicon semiconductors in an i-type semiconductor layer is lower than that of amorphous silicon solar cells with the same structure. The results make clear that the main cause lies in the interfaces between the n-type semiconductor layer or the p-type semiconductor layer with the i-type semiconductor layer. Specifically, the present inventors have discovered that there are many defect states in the vicinity of the interface of the n-type semiconductor layer with the i-type semiconductor layer as well as in the vicinity of the interface of the p-type semiconductor layer with the i-type semiconductor layer, which function as recombination centers. The existence of the recombination centers results in reduction in number and lowering in transportability of photo-excited free carriers in the i-type semiconductor layer. As a result, the open circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF) of the solar cell decline. Further, it is attributable to an increase in series resistance and a decline in the shunt resistance of the solar cell. As a result, the conversion efficiency of the solar cell declines.
When the inventors teamed up a transmission electron microscope with a secondary ion mass spectrometer and searched for the cause of the many defect states in the vicinity of the interfaces mentioned above, they discovered that the n-type semiconductor layer and the i-type semiconductor layer, or the p-type semiconductor layer and the i-type semiconductor layer were discontinuously stacked. Thus, they assumed that the reason why there were many defect states in the vicinity of the interfaces mentioned above was that the n-type semiconductor layer and the i-type semiconductor layer, or the p-type semiconductor layer and the i-type semiconductor layer were discontinuously stacked.
Further, when usual semiconductor elements are left to stand in the atmospheric environment, molecules in the air (water, oxygen, nitrogen, nitrogen oxides, sulfurous compounds, etc.) or the elements contained therein may sometimes diffuse into the semiconductor element to lower the characteristics of the semiconductor element. Similarly, when a semiconductor element such as a solar cell is encapsulated with another material (encapsulant), a chemical substance (acetic acid, etc.) contained in the encapsulant may sometimes diffuse into the semiconductor element to lower the characteristics of the semiconductor element. In particular, when each layer is stacked discontinuously at the semiconductor junction (junction of n-type semiconductor layer with i-type semiconductor layer, junction of p-type semiconductor layer with i-type semiconductor layer, etc.), the diffused substance will be trapped by the interface defects to lower the semiconductor characteristics.
When the inventors analyzed transmission electron microscope and X-ray diffraction data, it became clear that structural distortions were liable to be concentrated in the relatively large spaces between microcrystal grains, where there were many defects. These defects will reduce the transportability (mobility) of photo-excited free carriers and shorten the lifetime thereof to lower the characteristics of the semiconductor element.
The present invention aims to solve the above-mentioned problems and improve the photoelectric conversion efficiency of a photoelectric conversion element represented by a solar cell.
The present invention also aims to eradicate the discontinuity in the semiconductor junction portion to thereby provide a semiconductor element with superior semiconductor characteristics.
The present invention further aims to reduce the defects between microcrystal grains and to dissolve the discontinuity at the semiconductor junction portion to thereby provide a semiconductor element with superior semiconductor characteristics.
In addition, the present invention aims to improve the heat resisting properties and durability of a semiconductor element.